Estimating the energetic cost of fighting in shore crabs by noninvasive monitoring of heartbeat rate

After establishing shore crabs, Carcinus maenas, individually in separate a quaria, we used a noninvasive infrared phototransducer to monitor their hea rtbeat rate continously before, during and after fights with intruder crabs . We confirmed that heartbeat rate is a reliable indicator of oxygen consum ption and then used it to estimate indirectly the energetic cost of fights differing in duration and intensity, and its dependence on prior residence and relative size of opponent. Prior residence in aquaria significantly inc reased the probability that crabs would initiate fights against intruders. The majority of fights were resolved by aggressive contacts, display being used extensively only against smaller intruders. Fights between evenly size d opponents and between residents and larger intruders involved almost cont inuous aggression, whereas fights with smaller intruders involved several s horter bouts of aggression. Fight duration was weakly correlated with the r elative size of opponents. Heartbeat rate, measured only in residents, was elevated above resting levels throughout fights, hence energy expenditure d uring fighting increased linearly with fight duration. Contrary to expectat ion, heartbeat rate was not significantly influenced by relative size of th e opponent or by the intensity of aggression. After fighting, heartbeat rat e usually returned to resting levels within 30-60 min, recovery taking long er in fights against larger intruders, when the fight was always lost. We p ropose that prolonged elevation of heartbeat rate in residents that had los t to larger intruders represented a state df alertness, adaptive against im pending risks of resource loss or injury. (C) 2000 The Association for the Study of Animal Behaviour.